This invention relates to a process for selectively isomerizing olefinic hydrocarbons. More particularly, this invention relates to a process for selectively isomerizing olefinic hydrocarbons, such as normal amylene, utilizing a catalyst comprising low to medium concentrations of borosilicate with a binder comprising silica.
Oxygenates have been part of the United States gasoline strategy since the late 1970s. With the Clean Air Act Amendments of 1990, the demand for oxygenates are expected to increase even further. For example, starting in the winter months of 1992, gasoline containing 2.7 weight percent oxygen will have to be provided to approximately 40 metropolitan areas that have failed to meet carbon monoxide pollution standards. It is expected that in the near future, between 30 and 60 percent of the United States gasoline pool may be required to contain oxygenates. Current oxygenate production capacity is insufficient for meeting these requirements.
The most commonly used oxygenates today are methanol, ethanol, and methyl tertiary butyl ether (MTBE). Although methanol and ethanol have high blending octanes, problems with toxicity, water miscibility, high Reid Vapor Pressure (RVP), high nitrogen oxide emissions, lower fuel efficiency, and cost have dampened industry enthusiasm for these components. Partially as a result of the above, MTBE has become particularly attractive.
Homologues of MTBE such as ethyl tertiary butyl ether (ETBE) and methyl tertiary amyl ether (TAME) are also gaining industry acceptance. Moreover, commercial activity with respect to ETBE and TAME is expected to increase relative to MTBE, in view of the recent Environmental Protection Agency decision to reduce the RVP requirements for gasolines well below 9 psia, the blending RVP of MTBE.
Oxygenate production is not only limited by oxygenate plant capacity but by feedstock availability. MTBE and ETBE both utilize isobutylene as a feedstock. Isobutylene is generally supplied to an MTBE or ETBE facility in a petroleum refinery, from a fluid catalytic cracking unit (FCC), a fluidized or delayed coker, or from downstream paraffin dehydrogenation and isomerization facilities. The availability of hydrocarbons having 4 carbons is limited by constraints such as, but not limited to, crude properties, FCC catalyst properties, FCC operating conditions, and coking conditions, etc. The chemical mix of C.sub.4 paraffins, olefins, and aromatics as well as the particular mix of iso-olefins to normal olefins are similarly constrained.
Once lower cost alternatives for expanding isobutylene availability have been exhausted, refiners are being faced with the decision of locating additional feed sources or erecting downstream facilities such as dehydrogenation and isomerization units, to convert normal butanes to isobutylene. Such facilities, which were not in commercial demand prior to such recent legislation, are particularly expensive to erect. Therefore, some refiners have opted for the production of TAME to provide incremental oxygenate supply, in view of problems associated with limited isobutylene availability and reduced gasoline RVP requirements. TAME utilizes isoamylene as a feedstock. However, isoamylene supply is subject to many of the same availability constraints as isobutylene.
Thus, there exists a great need in the petroleum industry for a method of increasing oxygenate production capacity. An obstacle to fulfilling this need is the limited availability of isoamylene feedstocks, necessary for the production of TAME.
Processes for isomerizing alkenes, and particularly normal butylene, are generally known in the art. The products of alkene isomerization have been used to provide reactants for polymerization, alkylation, and disproportionation reactions in addition to MTBE production.
U.S. Pat. No. 4,038,337 to Manara et al. discloses a process for the skeletal isomerization of alkenes utilizing a catalyst comprising alumina that has been treated with minor amounts of silica.
U.S. Pat. No. 4,548,913 discloses a process for the skeletal isomerization of alkenes utilizing a catalyst generally comprising zeolites at silicon to aluminum molar ratios of 300:1 to 5000:1.
The use of borosilicate-containing catalysts for oligomerization, isomerization, and aromatization has also been taught in the art.
U.S. Pat. No. 4,777,310 to Sikkenga discloses a process for selective gas-phase equilibration of olefins having 3 carbon atoms or more using a catalyst comprising a borosilicate molecular sieve and an inorganic matrix. An objective of the process is the maximization of total butylene and t-amylene production and minimization of aromatics. Process objectives are achieved by maintaining a particularly specified partial pressure of monoolefin feed and short catalyst and oil contact times.
U.S. Pat. No. 4,503,282 to Sikkenga discloses a process for converting substantially linear alkenes such as n-butene, to isomerized products using a catalyst comprising greater than 50 weight percent of a borosilicate molecular sieve with an inert binder. An objective of the process is to provide iso-butylene feedstocks for subsequent processing steps such as polymerization and oxidation.
U.S. Pat. No. 4,499,325 to Klotz et al. discloses a process for converting alkenes to oligomerized, aromatized, or isomerized products using a borosilicate catalyst composition. The process provides for a method to convert linear butenes to a mixture containing isobutylene, a method to dimerize the isobutylene using the borosilicate catalyst composition, and a method to convert the butenes to aromatics.
U.S. Pat. No. 4,499,326 to Melquist discloses a process for the isomerization of normal butene to isobutylene using a borosilicate catalyst composition and low reaction temperatures. An objective of the process is to provide a process that produces 2-butene from 1-butene for feedstock to an alkylation process so as to produce a higher octane alkylate product.
A problem attendant to such processes is that under field or commercial process conditions, such catalysts generally require periodic regeneration to maintain catalytic activity and selectivity. Upon experiencing regeneration conditions, which can include exposure to high temperatures and steam formed from the combustion of coke, such catalysts can experience substantial losses in isomer yield.
It has been found that under simulated regeneration conditions, typical borosilicate-containing catalysts, such as those described hereabove, can undergo a substantial loss in i-Amylene selectivity when utilized for n-Amylene isomerization. It has also been found that utilizing reduced amounts of crystalline borosilicate molecular sieve (less than 50 weight percent) combined with a silica binder, provides substantially improved i-Amylene yield performance, under commercial conditions, than borosilicate-containing catalysts having larger proportions of the crystalline borosilicate molecular sieve or borosilicate-containing catalysts comprising inorganic oxide binders other than silica.
It has also been found that the catalyst described herein, comprising reduced concentrations of crystalline borosilicate molecular sieve with a silica binder, provides even better performance when utilized under high reaction temperatures (between about 500.degree. F. and 1200.degree. F.) at a particularly targeted n-Amylene-containing feedstock space velocity (WHSV) of between about 4 hr.sup.-1 and about 32/hr.sup.-1. Lower space velocities generally result in i-Amylene selectivity losses that outpace the gains in n-Amylene conversion, resulting in lower i-Amylene yields. Higher space velocities result in losses in n-Amylene conversion that outpace the gains in i-Amylene selectivity, resulting in lower i-Amylene yields.
For purposes of the present invention, normal amylene conversion (n-Amylene Conv.), isoamylene selectivity (i-Amylene Sel.), and isoamylene yield (i-Amylene Yield) shall have the following meanings and be calculated by weight and in accordance with the following models: ##EQU1##
It is therefore an object of the present invention to provide a process that cost-effectively increases oxygenate production capability by increasing i-Amylene feedstock availability.
It is another object of the present invention to provide a process that can be operated reliably and effectively under conditions wherein the catalyst undergoes periodic regeneration.
It is another object of the present invention to provide a process that can easily accommodate a stand alone operating facility or incorporation with existing process units.
It is yet another object of the present invention to provide a process that can operate effectively in the presence of or without hydrogen.
Other objects appear herein.